Methods for transmitting and receiving laser signals, as well as transmitter and receiver which carry out said methods

ABSTRACT

Method for transmitting laser signals (L), in particular in the Free Space Optics (FSO) telecommunication systems, wherein an electric signal (S) containing information to be transmitted is received by an input interface (IN) and sent to a laser emitter (LE) which emits a laser signal (L) containing said information, wherein before reaching the laser emitter (LE) said electric signal (S) orone or more electric signals (I, Q) corresponding thereto modulate one or more carrier signals (CS, sin cat, cos wt) at intermediate frequencies comprised between the frequencies of the laser signal (L) and the electric signal (S) received by the input interface (III. The present invention also relates to a method for receiving the laser signals so transmitted, as well as a transmitter and a receiver which carry out said methods.

The present invention relates to a method for transmitting laser signals, and in particular to a method which can be employed for transmitting laser signals in free space (air). The most common applications comprise for instance telecommunication, ATM or Ethernet/IP networks, as well as surveillance or broadcast service. systems, etc. The present invention also relates to a method f6r receiving the laser signals so transmitted, as well as a transmitter and a receiver which carry out said methods.

The known methods and apparatuses for transmitting and receiving laser signals provide for a direct modulation of the laser signal by means of an analog or digital signal containing the information to be transmitted, as well as a corresponding direct demodulation of the received laser signal. Some known kinds of direct modulation are the OOK (On/Off Keying) modulation and the PPM (Pulse Position Modulation) modulation.

In these known methods the transmitter works in saturation conditions (ON/OFF) and employs the electric signal containing the data to be transmitted for directly modulating a laser signal, which comprises a corresponding sequence of more or less intense photon emissions. Analogously, the receiver transforms the received laser signal into an electric signal which is equalized in amplitude and synchronized for returning the electric signal of the transmitter.

The optical communication systems in free space, called Free Space Optics (FSO), are generally based on one or more laser emitters coupled with one or more lenses or optical devices in the transmitter, and on one or more lenses for focusing the received laser beam onto a PIN- or APD-type electro-optical transducer/detector in the receiver. In this known optical communication method, artificial and/or natural light interferences are translated in the receiver as low frequency interfering signals which overlap the transmitted signal, thereby altering the signal/noise ratio in the receiver and decreasing the performances of the transmission system.

Known methods and apparatuses for directly modulating the laser signal with high frequency carrier signals are described in the articles “Simultaneous electro-optical upconversion to 60 GHz of uncoded OFDM signals” of M. Sauer et al. (XP010319747) and “Optical interfaces without light sources for base station designs in fiber-wireless systems incorporating WDM” of A. Nirmalathas et al. (XP010367434).

U.S. Pat. No. 5146359 disclose instead known laser signal receivers, which carry out the demodulation of these.signals by means of complex methods and devices which are not particularly suitable for eliminating light interferences, since they are essentially dedicated to the reception of laser signals transmitted through optical fibers.

It is therefore an object of the present invention to provide methods and apparatuses which are free from said disadvantages, i.e. which allow to transmit and receive laser signals with better performances not only through optical fibers, but also and especially in a free space. Said object is achieved with two methods, a transmitter and a receiver, the main features of which are disclosed in claims 1, 12, 19 and 29, respectively, while other features are disclosed in the remaining claims.

In the transmission method and in the transmitter according to the present invention the analog or digital signal to be transmitted is not used as in the known methods for directly modulating the signal of a laser emitter, but undergoes at least one modulation with a carrier signal having an intermediate frequency before it is sent thereto. Analogously, in the reception method and in the receiver according to the present invention the electric signal obtained from the laser receiver is demodulated and decoded, if necessary, for obtaining the original signal. The analog or digital signal to be transmitted is therefore employed for modulating one or more carrier signals at an intermediate frequency which in turn modulate the laser signal. The present invention allows to use more modulated signals combined with each other which directly control the laser signal, provided that a homogeneous modulation process for all signals is used, so as to exploit the whole dynamic of the electric signal controlling the laser signal.

Thanks to the particular modulation and demodulation methods, the present invention also allows the use of:

-   selective filterings of the intermediate frequency signal; -   automatic gain control circuits which increase the dynamic of the     received signal; -   specific algorithms for error correction, signal scrambling, as well     as bit and symbol interleaving, so as to notably improve the system     gain, i.e. the signal/noise ratio in the receiver, with respect to     an non-encoded system.

The electric signal to be transmitted is suitably modulated at an intermediate frequency, for instance of 70 or 140 MHz, which depends upon the kind of signal to be transmitted, in particular by using modulation processes of the FSK (Frequency Shift Keying), PSK (Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), OQPSK (Offset Quadrature Phase Shift Keying) or OQAM (Offset Quadrature Amplitude Modulation) kind for digital signals or of the FM (Frequency Modulation) kind for analog signals, which allow the use of circuits working in saturation at a constant power.

In these known kinds of modulation at constant amplitude, contrary to the known methods and apparatuses which use OOK and/or PPM kind modulations, wherein the information to be transmitted depends upon the amplitude and/or the position of the signal pulse, the information depends upon its phase or frequency. The information can thus be easily retrieved since it is totally insensible to the use of circuits and devices which work in saturation.

It is also possible to employ modulation processes which provide for the use of linear circuits as the M-QAM Multilevel Quadrature Amplitude Modulation) and COFDM (Coded Orthogonal Frequency Division Modulation) processes for digital signals or more simply AM (Amplitude Modulation), AM-DSB (Amplitude Modulation Double Side Band), AM-DSB-SC (Amplitude Modulation Double Side Band-Suppression Carrier), AM-VSB (Amplitude Modulation Vestigial Side Band) processes for analog signals, although with a decrease of the usable signal/noise ratio in the receiver and therefore of the availability of the laser connection.

The methods and the apparatuses according to the present invention allow to obtain considerable advantages with respect to the known ones, and in particular:

1) they are less sensible to natural or artificial optical interferences at constant or variable power (sun, gas discharge lamps, etc.) which are converted into a low frequency noise in the receiver at a band going from the continuous component up to about 1-2 MHz, since the transmission of the modulated signal is substantially free from the continuous component;

2) they limit the signal band, so that they can combine different signals at intermediate frequencies which contain different information but use modulation processes homogeneous with each other;

3) in case of digital signals, they can use error correctors of the FEC (Forward Error Correction) kind associated to bit and symbol interleaving algorithms for fighting the selective fading (scintillation) due to atmospheric phenomena and linked to the propagation of the laser signal in air, thus increasing the reliability and above all the connection availability,

4) by using electro-optical devices and components for high bit rates (1,25 Gb/s and above), they can combine digital signals modulated at different intermediate frequencies, for example with FDM (Frequency Division Multiplexing) multiplexing techniques, so as to increase the capacity of the transmitted data coming from different sources and sent through a single laser connection;

5) by reducing the power of the modulating signal of the laser emitter, they can transmit analog signals modulated in amplitude or digital signals encoded on more levels by means of a mono-carrier or multi-carrier modulator.

By decreasing the system gain of the system, the method according to the. present invention further allows the distribution of data and broadcast services for public or private networks at high bit rates by using optical systems in free space on distances from 0,2 to 1 Km.

According to a particular aspect of the invention, the intermediate frequency of the carrier signals corresponds to the speed of the encoded and decoded data in the transmitter and in the receiver, respectively, so as to simplify their working. In particular the working of the receiver is simplified, since the frequency of the encoded and demodulated digital signals coincides with the clock frequency of the same receiver. With this arrangement it is possible to employ a single clock recovery algorithm which automatically recovers also the carrier signal.

Therefore, the sequence of the base band symbols which carry the information is modulated in the transmitter by means of a carrier signal, the frequency of which is linked to the frequency of the symbols of the same signal, and is subsequently demodulated in the receiver without the necessity of estimating the phase and the frequency of the carrier signal, since it coincides with the frequency of the clock recovered from the demodulated information. All this involves an improvement of the features of the demodulator with respect to non-encoded systems and demodulators, in which the performances referred to the error rate (Bit Error Rate) are characterized by the performances of the algorithm which estimates and recovers the phase and the frequency of the carrier signal. In the present invention, the use of the sole clock recovery algorithm in the reception, which is simultaneously employed also as a carrier signal, increases the performances under the same noise in terms of resistance to the carrier droppings when there is a low signal/noise ratio, i.e. very high error rates, for instance from 1*10⁻³ to 1*10⁻².

Further advantages and features of the methods, the transmitter and the receiver according to the present invention will become clear to those skilled in the art from the following detailed and non-limiting description of two embodiments thereof with reference to the attached drawings, wherein:

-   FIG. 1 shows a block scheme of a first digital transmitter which     carries out the transmission method according to the first     embodiment of the invention; -   FIG. 2 shows a block scheme of a first digital receiver which     carries out the reception method according to the first embodiment     of the invention; -   FIG. 3 shows a block scheme of a second digital transmitter which     carries out the transmission method according to the second and     preferred embodiment of the invention; and -   FIG. 4 shows a block scheme of a second digital receiver which     carries out the reception method according to the second and     preferred embodiment of the invention.

Referring to FIG. 1, it is seen that the transmitter according to the first embodiment of the invention comprises in a known way an input interface IN which receives the signal S to be transmitted and adapts it according to the kind of signal. For example, if the signal S is digital, interface IN adapts its bit rate and code, while if the signal S is analog, interface IN adapts it in amplitude and determines the input impedance of the transmitter. Interface IN extracts the clock CK from the data of signal S and is connected to an encoding circuit EC comprising in particular a scrambler SC which encodes the signal DT coming from interface IN so that the transmitted signal has in the frequency domain a homogeneous and continuous spectrum ensuring a constant average power. Said encoding is always active also when there is no input signal for facilitating the signal locking operations in the receiver. Scrambler SC is connected to an encoder RSE, in particular of the Reed-Solomon kind, for correcting errors, in any, in the signal, which is in turn connected to an interleaver IL, for instance of the convolutional kind, for attenuating the burst errors. Interleaver IL is connected to an encoder EN, in particular of the FEC (Forward Error Correction) kind, which encodes the signal by adding a redundance word code depending upon the selected rate (for example with a rate of 1/2 encoder EN inserts a redundance bit every bit of transmitted data), thus preparing the signal for a mapper MA which, according to the signal received from the previous circuits and algorithms, to the working mode and to the chosen configuration, builds a data constellation, for instance according to the Gray mapping, and sends a corresponding pair of signals I and Q containing the symbols to be transmitted.

Signals I and Q encoded by the encoding circuit EC are filtered by a pair of digital filters DF, in particular of the FIR (Finite Impulse Response) kind, which, according to the cases, can act as anti-alias filters or shape the signal pulse with a determined roll-off. After this channel filtering operation, signals I and Q are converted into analog signals by a pair of digital-to-analog converters DAC and filtered again by a pair of analog filters AF also acting as anti-alias filters. The clock signal CK transmitted by interface IN is employed for synchronizing the devices of the encoding circuit EC, the digital filters DF and the digital-to-analog converters DAC.

After said encoding and filtering phase, signals I and Q obtained by the input signal S come to a pair of modulators MD, which suitably modulate these signals with a pair of sinusoidal and cosinusoidal carrier signals of the sin ωt and cos ωt kind, obtained by shifting of 90° with a phase shifter PS a carrier signal CS which in the present embodiment consists of a wave generated by a free oscillator OSC.

The I*cos ωt and Q*sin ωt signals sent by modulators MD are added to each other by an adder SUM and the resulting modulated signal M is sent to an amplifier TA. The latter amplifies in a known way the signal M before sending it to the driver LD of a laser emitter LE of a known kind which emits, for instance through the free space, a laser signal L corresponding to this electric signal. Driver LD can comprise an auxiliary output AO for carrying out local checks of the signal sent by the laser emitter LE or for sending the signal to a second laser emitter, if any.

The frequency of the carrier signals CS, sin ωt and cos ωt is suitably an intermediate frequency, for instance of 70 MHz, comprised between the frequency of the input signal S and the frequency of the laser signal L, in particular comprised between 10 and 2000 MHz. Amplifier TA can comprise a band-pass filter BPF set at this intermediate frequency for eliminating harmonics, if any, generated by modulators MD.

Between the band-pass filter BPF and driver LD can be arranged a combiner CO which combines in a known way signal M with other independent signals M′ at different intermediate frequencies coming from other circuits with modulators (not shown in the figure) similar or the same as the above described one, thus obtaining a single signal C which contains these signals and is sent to the laser emitter LE.

Referring now to FIG. 2, it is seen that the receiver according to the present embodiment of the invention comprises in a known way a laser receiver LR, for example a detecting photodiode of the PIN or APD kind, which receives a laser signal L, for instance transmitted by the above described transmitter. The laser receiver LR sends a corresponding electric signal D to a high dynamic intermediate frequency amplifier IFA with variable gain, which is preferably connected to an AGC (Automatic Gain Control) circuit for increasing the dynamic of the received signal. Amplifier IFA provides a signal N of the I*sinωt+Q*cos ωt kind to a pair of demodulators DM which demodulate this signal according to two carrier signals sin ωt and cos ωt obtained by phase shifting with a phase shifter PS a carrier signal CS which in the present embodiment consists of a wave generated by a voltage controlled oscillator VCO having the same frequency of the free oscillator OSC contained in the transmitter of the first embodiment. Circuit AGC is preferably set at the intermediate frequency of signal N by means of a band-pass filter connected upstream the same circuit. Between amplifier FA and demodulators DM can be arranged a splitter SP which splits signal D by separating a signal N from a plurality of other independent signals N′ at different intermediate frequencies combined with each other in the transmitter which transmitted signal L, for instance in the above described transmitter. In this case, splitter SP is connected to a band-pass filter BPF which is set at the intermediate frequency of signal N and is in turn connected to a wide-band buffer WBB connected to demodulators DM. The intermediate frequency signals N′ are sent to other circuits with demodulators (not shown in the figure) for obtaining electric signals corresponding thereto.

The demodulated signals I and Q are filtered by a pair of low-pass filters LPF and are amplified by a pair of base band amplifiers BBA which are preferably high dynamic amplifiers with variable gain and are connected to a circuit AGC (Automatic Gain Control) for keeping the amplitude of the received signal constant. Signals I and Q amplified by amplifiers BBA are then converted into a digital form by a pair of analog-to-digital converters ADC and sent to a pair of digital filters DF, in particular of the FIR (Finite Impulse Response) kind, which act also as adaptive equalizers of signals I and Q. In fact, filters DF control the quality of the demodulated signals I and Q and adjust in real time the values of the coefficients of their inputs. The signals so filtered and equalized are therefore free from ISI (Inter Symbol Interference) interference and errors due to phenomena caused by the propagation of the laser signal L in air, such as for example the selective fading (scintillation) present in particular atmospheric conditions. The information correlation between signals I and Q further allows, by means of a control circuit CC, to obtain the correct voltage for controlling the frequency and the phase of oscillator VCO and thus reconstruct the carrier signal CS of the transmitter, in particular the signal generated by the free oscillator OSC of the above described transmitter. The output signal from filters DF are subsequently processed by a decoding circuit DC comprising a demapper DMA connected to a decoder DE, for instance of the FEC (Forward Error Correction) kind, which corrects errors, if any, in the signals and provides a single signal S. The latter is in turn reordered by a deinterleaver DIL, decoded by a decoder RSD, for example of the Reed-Solomon kind, and by a descrambler DSC, as well as transmitted to an output interface OUT, which returns a signal S, in particular the same as the one received from the input interface IN of the above described transmitter. The interface OUT returns the data in a suitable format in terms of bit rate and code for being correctly sent outside, for example toward a data network or an electronic device. In case of an analog signal, the output interface OUT can provide the signal with amplitude and impedance as defined in the ITU-T and/or ETSI standards. The signals I and Q transmitted by the digital filters DF are also sent to a clock recovery circuit CR, also of the known kind, which recovers the clock signal CK of the symbols transmitted through the signals I and Q, so as to synchronize the decoding circuit DC, the same digital filters DF and the digital-to-analog converters DAC.

Referring to FIG. 3, it is seen that the second and preferred embodiment of the transmitter is similar to the first one, but differs in that the frequency of the carrier signals CS, sin cot and cos ωt does not depend upon oscillator OSC, but is suitably an intermediate frequency corresponding to the speed of the data in signal S, i.e. to the frequency of the relevant clock signal CK. For this purpose, interface IN transmits the clock signal CK also to a multiplier MUL which multiplies this signal by a factor depending upon the code rate used in the encoding circuit EC. Multiplier MUL sends then to the phase shifter PS a carrier signal CS at a frequency f(CS) which is for instance determined according to the following formula: f(CS)=f(CK)×r(RSE)×r(EN) wherein f(CS) is the frequency of the carrier signal CS, f(CK) is the frequency of the clock signal CK, R(RSE) is the redundancy introduced by encoder RSE and r(EN) is the redundancy introduced by encoder EN.

Therefore, for obtaining the frequency f(CS) of signal CS the frequency f(CK) of the clock signal CK is multiplied by a factor X/Y which can be an integer or fractional number which depends upon the kind of encoding applied to the input signal S.

The devices of the encoding circuit EC, of the digital filters DF and of the converters DAC are controlled by a clock signal CS′ which is generated by multiplier MUL and can be equal to the clock signal CK, to the carrier signal CS or to a multiple thereof.

Referring to FIG. 4, it is seen that the second and preferred embodiment of the receiver is similar to the first one, but differs in that the carrier signals sin ωt and cos ωt are obtained by shifting with the phase shifter PS a carrier signal CS obtained not from a VCO oscillator, but from a multiplier MUL which according to the clock signal CK received from the clock recovery circuit CR obtains the signals for the working of demodulators DM and for the decoding circuit DC. Multipliers MUL of the receiver and of the transmitter according to the second embodiment therefore work substantially in the same way.

In the various embodiments of the present invention, the signal to be transmitted can be modulated with modulation processes of the FSK (Frequency Shift Keying), PSK (Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), OQPSK (Offset Quadrature Phase Shift Keying) or OQAM (Offset Quadrature Amplitude Modulation) kind for digital signals or of the FM (Frequency Modulation) kind for analog signals, so as to employ optoelectronic circuits and devices which work in saturation conditions.

For example, the FM modulation can be employed for transmitting a broadcast analog video signal with optical transceivers in the free space, while the other above cited modulations are suitable for the digital transmission of data for telephone telecommunication networks or packed digital transmission in ATM networks and/or with IP/Ethernet protocols.

In other embodiments of the present invention, the signal to be transmitted can be modulated with modulation processes of the M-QAM (Multilevel Quadrature Amplitude Modulation) and COFDM (Coded Orthogonal Frequency Division Modulation) kind for digital signals or of the AM (Amplitude Modulation), AM-DSB (Amplitude Modulation Double Side Band), AM-DSB-SC (Amplitude Modulation Double Side Band-Suppression Carrier), AM-VSB (Amplitude Modulation Vestigial Side Band) kind for analog signals, so as to employ optoelectronic circuits and devices which work in a linear state.

The present invention can be applied to all the transmitters and receivers which can transmit analog and digital signals exploiting wavelengths suitable for the transmission in the free space (air), i.e. at 780, 850, 870, 911 nm (₁st window), 1300, 1310 nm (2^(nd) window) and 1550 nm (3^(rd) window) with powers from some mW to some hundreds of mW. 

1. Method for transmitting laser signals (L) in free space, wherein an electric signal (S) containing information to be transmitted is received by an input interface (IN) and sent to a laser emitter (LE) which emits a laser signal (L) containing said information, characterized in that before reaching the laser emitter (LE) said electric signal (S) or one or more electric signals (I, Q) corresponding thereto modulate one or more carrier signals (CS, sin cot, cos cot) at intermediate frequencies comprised between the frequencies of the laser signal (L) and the electric signal (S) received by the input interface (IN), wherein the electric signal (S), before it modulates the carrier signals (CS, sin cω, cos cω), is encoded for obtaining one or more electric signals corresponding thereto (I, Q).
 2. Method according to the previous claim, characterized in that the electric signal (M) modulated in said modulation of the carrier signals (CS, sin cω, cos cω) at an intermediate frequency is amplified before it reaches the laser emitter (LE).
 3. Method according to claim 1 or 2, characterized in that the electric signal (M) modulated at an intermediate frequency is combined with other electric signals (M′) modulated at intermediate frequencies before it reaches the laser emitter (LE).
 4. Method according to one of the previous claims, characterized in that the encoding of said electric signal (S) comprises processes of scrambling, Reed-Solomon encoding, interleaving and/or FEC encoding.
 5. Method according to one of the previous claims, characterized in that the signal or the signals (I, Q) obtained from said encoding are filtered and converted into an analog form before they are modulated.
 6. Method according to one of the previous claims, characterized in that the carrier signals (CS, sin cω, cos cω) are modulated with one or more digital signals (I, Q) by means of a FSK, PSK, QPSK, OQPSK or O-QAM modulation process at a constant power or by means of a QAM, M-QAM or COFDM modulation process for digital linear signals.
 7. Method according to one of claims 1 to 5, characterized in that the carrier signals (CS, sin cω, cos cω) are modulated with one or more analog signals by means of a FM modulation process at a constant power or by means of a AM, AM-DSB, AM-DSB-SC or AM-VSB modulation process with amplitude envelope.
 8. Method according to one of the previous claims, characterized in that the carrier signals (CS, sin cω, cos cω) have a frequency comprised between 10 and 2000 MHz.
 9. Method according to one of the previous claims, characterized in that the intermediate frequency of the carrier signals (CS, sin cot, cos cot) corresponds to the frequency of the clock signal (CK) contained in the signal (S) to be transmitted multiplied by a factor depending upon the kind of encoding applied to this signal (S).
 10. Method according to claim 9, characterized in that the intermediate frequency of the carrier signals (CS, sin cot, cos cot) is obtained from the following formula: f(CS)=f(CK)×r(RSE)×r(EN) wherein f(CS) is said intermediate frequency, f(CK) is the frequency of said clock signal (CK), r(RSE) is the redundancy introduced by a Reed-Solomon encoding and r(EN) is the redundancy introduced by a FEC encoding.
 11. Method for receiving laser signals in free space, wherein a laser signal (L) containing information is received by a laser receiver (LR) and sent to an output interface (OUT) which returns an electric signal (S) containing said information, characterized in that at least one electric signal (D) corresponding to said laser signal (L) is demodulated by means of one or more carrier signals (CS, sin ωt, cos ωt) at intermediate frequencies comprised between the frequencies of the laser signal (L) and of the electric signal (S) returned by the output interface (OUT), after which it is sent to the output interface (OUT), wherein the signals (I, Q) obtained by said demodulation are decoded before they reach the output interface (OUT).
 12. Method according to claim 11, characterized in that the electric signal (D) corresponding to the laser signal (L) is amplified by one or more variable gain amplifiers (IFA, AGC) before it is demodulated.
 13. Method according to claim 11 or 12, characterized in that the electric signal (D) corresponding to the laser signal (L) is divided by a splitter (SP) for obtaining a plurality of electric signals (N, N′) at intermediate frequencies.
 14. Method according to one of claims 11 to 13, characterized in that the decoding of said electric signals (I, Q) comprises processes of FEC decoding, deinterleaving, Reed-Solomon decoding and/or descrambling.
 15. Method according to one of claims 11 to 14, characterized in that said electric signals (I, Q) are filtered and converted into a digital form before they are decoded.
 16. Method according to claim 14 or 15, characterized in that the intermediate frequency of the carrier signals (CS, sin cot, cos cot) corresponds to the frequency of the clock signal (CK) contained in said received electric signals (I, Q) multiplied by a factor depending upon the kind of decoding applied to these signals (I, Q).
 17. Transmitter for laser signals in free space, which comprises at least one input interface (IN) suitable for receiving an electric signal (S) containing information to be transmitted, as well as a laser emitter (LE) which emits a laser signal (L) containing said information, characterized in that one or more modulators (MD), suitable for modulating with said electric signal (S) or with one or more electric signals (I, Q) corresponding thereto one or more carrier signals (CS, sin cot, cos cot) at intermediate frequencies comprised between the frequencies of the laser signal (L) and of the electric signals (S) received by the input interface (IN), are arranged between the input interface (IN) and the laser emitter (LE), wherein the input interface (IN) is connected to an encoding circuit (EC) which encodes the electric signal (S) containing the information to be transmitted and provides one or more encoded signals (I, Q) which modulate the carrier signals (CS, sin cot, cos cot).
 18. Transmitter according to claim 17, characterized in that the encoding circuit (EC) comprises a scrambler (SC), a Reed-Solomon encoder (RSE), an interleaver (IL) and/or a FEC encoder (EN).
 19. Transmitter according to claim 17 or 18, characterized in that one or more electric signals (I, Q) containing the information to be transmitted are filtered by one or more digital filters (DF) of the FIR kind before they modulate the carrier signals (CS, sin ωt, cos ωt).
 20. Transmitter according to one of claims 17 to 19, characterized in that one or more signals emitted by the modulators (MD) are added to each other by an adder (SUM).
 21. Transmitter according to one of claims 17 to 20, characterized in that the modulated signal (M) to be transmitted is amplified by an amplifier (TA) connected to the laser emitter (LE).
 22. Transmitter according to claim 21, characterized in that said amplifier (TA) comprises a band-pass filter (BPF) set at the intermediate frequency of the carrier signals (CS, sin ωt, cos ωt).
 23. Transmitter according to one of claims 17 to 22, characterized in that a combiner (CO) which combines the modulated signal (M) with other signals (M′) modulated at different intermediate frequencies is arranged before the laser emitter (LE), so as to obtain a single signal (C).
 24. Transmitter according to one of claims 17 to 23, characterized in that the input interface (IN) extracts a clock signal (CK) from the signal (S) to be transmitted and sends it to a multiplier (MUL) which multiplies this signal by a factor depending upon the code rate used by the encoding circuit (EC), so as to obtain said carrier signals (CS, sin ωt, cos ωt).
 25. Transmitter according to claim 24, characterized in that the intermediate frequency of the carrier signals (CS, sin ωt, cos ωt) is obtained from the following formula: f(CS)=f(CK)×r(RSE)×r(EN) wherein f(CS) is said intermediate frequency, f(CK) is the frequency of said clock signal (CK), r(RSE) is the redundancy introduced by the Reed-Solomon encoder (RSE) and r(EN) is the redundancy introduced by the FEC encoder (EN).
 26. Receiver for laser signals in free space, which comprises at least one laser receiver (LR) suitable for receiving a laser signal (L) containing information, as well as an output interface (OUT) suitable for returning an electric signal (S) containing said information, characterized in that one or more demodulators (DM), suitable for demodulating one or more electric signals corresponding to said laser signal (L) with one or more carrier signals (CS, sin ωt, cos ωt) at intermediate frequencies comprised between the frequencies of the laser signal (L) and of the electric signal (S) returned by the output interface (OUT), are arranged between the laser receiver (LR) and the output interface (OUT), wherein the electric signals (I, Q) returned by said demodulators (DM) are decoded by a decoding circuit (DC).
 27. Receiver according to claim 26, characterized in that the decoding circuit (DC) comprises a FEC decoder (DE), a deinterleaver (DIL), a Reed-Solomon decoder (RSD) and/or a descrambler (DSC).
 28. Receiver according to claim 26 or 27, characterized in that the signals (I, Q) returned by said demodulators (DM) are filtered by one or more digital filters (DF) of the FIR kind.
 29. Receiver according to claim 28, characterized in that said digital filters (DF) are connected to a control circuit (CC) which provides a control signal to at least one voltage controlled oscillator (VCO) which provides one or more carrier signals (CS, sin cot, cos cot) at intermediate frequencies according to this control signal.
 30. Receiver according to one of claims 26 to 29, characterized in that the carrier signals (CS, sin cot, cos cot) at intermediate frequencies are obtained from a multiplier (MUL) according to a clock signal (CK) received from a clock recovery circuit (CR).
 31. Receiver according to claim 30, characterized in that the clock recovery circuit (CR) is connected to said digital filters (DF) for obtaining a clock locking signal (CK) contained in said electric signals (I, Q).
 32. Receiver according to one of claims 26 to 31, characterized in that at least one electric signal (D) corresponding to the laser signal (L) is amplified by at least one variable gain amplifier (IFA, AGC) connected to the demodulators (DM).
 33. Receiver according to claim 32, characterized in that said variable gain amplifier (IFA, AGC) is preferably set at the intermediate frequency of the received signal (N) by means of a band-pass filter (BPF).
 34. Receiver according to one of claims 26 to 33, characterized in that a splitter (SP), which divides the electric signal (D) corresponding to the laser signal (L) and separates a plurality of mutually combined independent signals (N, N′) at different intermediate frequencies, is arranged before the demodulators (DM).
 35. Receiver according to claim 34, characterized in that the splitter (SP) is connected to a band-pass filter (BPF) which is set at the intermediate frequency of the received signal (N) and is in turn connected to a wide-band buffer (WBB). 